U.S. Pat. No. 4,777,915 discloses an electromagnetic actuator for operating a engine valve of a piston-type internal-combustion engine. The actuator has two spaced electromagnets, between the pole faces of which an armature is movable back and forth against the force of resetting springs. The armature is affixed to the stem of the engine valve. The upper electromagnet serves as a closing magnet while the lower electromagnet serves as an opening magnet, so that by an alternating energization of the closing magnet and the opening magnet the engine valve may be closed and opened. Since due to temperature changes and/or wear the opening and, in particular, the closing conditions for the valve and the predetermined valve clearance (valve slack) change, in such a system a piston-and-cylinder unit is provided which shifts the closing magnet in accordance with operational requirements whereby the valve clearance is changed. Such a solution, however, involves the disadvantage that by changing the setting of the valve clearance the stroke of the setting unit, that is, the motion path of the armature between the two pole faces of the electromagnets also changes which is impermissible in case of throttle-less load control in internal-combustion engines, because the charge quantity in the cylinder would thereby change as a function of temperature and time.
German Offenlegungsschrift (application published without examination) No. 197 02 485 discloses an electromagnetic actuator which includes a valve slack adjuster in which the electromagnets are arranged at a fixed distance from one another in a housing which also includes the valve opening spring. For performing valve slack adjustments, a hydraulic setting device is provided which is coupled with a hydraulic fluid supplying device by means of which the housing may be shifted relative to the engine valve coupled with the valve closing spring. By virtue of this arrangement a compensation of the valve clearance may be effected without changing the motion geometry of the armature and thus without changing the stroke of the setting unit. The housing is adjusted corresponding to temperature or wear-caused changes so that, for example, a chatter is avoided. Such a system, however, requires a significant technological and structural outlay since the setting unit must also serve for securing and guiding the actuator housing.
European Published Patent Application No. 814 238 discloses an electromagnetic actuator for a engine valve in which the bottom of the housing which surrounds the electromagnets has an axial guiding passage in which a conventional clearance compensating element, formed of a piston and a cylinder, is displaceably arranged. One end of the compensating element engages the outer end of the armature stem, while the other end of the compensating element engages the outer end of the valve stem. Since the clearance compensating element has a relatively large structural volume and, accordingly, a relatively large mass, this construction has the disadvantage that during valve operation the clearance compensating element is reciprocated with the operating frequency of the actuator. In view of the accelerations, the mass forces produced by the clearance compensating element are not negligible. Since such forces oppose the armature motion, a larger energy requirement is necessarily needed.